The customer judges the quality of paper products by evaluating a combination of properties, including smoothness, gloss, brightness and feel. In particular, tactile feedback indicating a higher caliper (thickness) for the paper conveys the impression that the product is of high quality. Making paper thicker, though, typically involves using more pulp, so that the paper is heavier and more expensive. To produce a thicker paper without incorporating additional pulp, the papermaking industry has identified a number of inexpensive particulate additives that act as low-density fillers to create the feel of a thicker paper while decreasing weight and cost. Examples include EXPANCEL® (Akzo Nobel) and OMNIBULK® (Kemira).
Such fillers, however, can impair the strength and resiliency of the final product, and do not hold up well under the forces applied during calendaring. Therefore, a need exists in the industry for formulations and methods to enhance the bulk of paper products while yielding sheets that remain strong and flexible. Desirably, such formulations and methods are compatible with existing papermaking techniques and equipment.
In addition to bulk, paper industry also desires an improvement in stiffness of paper products. There is particularly need for such improvement in packaging industry where rigidity is necessary for structural reasons and fine papers where rigidity is necessary to ensure that the paper can undergo repeated bending in copying and printing processes yet retain its dimensional stability.